1. Field of the Invention
This invention relates to a metal gasket for a manifold which is tightened under a clamped state between the end surface of an exhaust manifold and a cylinder head having open exhaust ports.
2. Description of the Prior Art
Conventionally, a metal gasket for an exhaust manifold of an engine which is clamped between the end surface of an exhaust manifold and the side surface of a cylinder head having open exhaust ports and tightened by fastening bolts or the like is disposed lest an exhaust gas leaks from the contact surface between the cylinder head and the exhaust manifold and various types of gaskets for a manifold have been employed.
A typical metal gasket for a manifold is of a type consisting of the laminate of metal plates. This metal gasket is produced generally by laminating three to five metal plates. In a metal plate laminate gasket having an odd-numbered metal plates such as a three-layered metal plate laminate gasket, for example, the metal plate at the center is composed of a flat steel sheet not having any bead and the metal plates on both sides are bead plates having a projection form so that their bead portions are spaced apart progressively from each other. Such a metal gasket is disclosed, for example, in Japanese Utility Model Laid-Open No. 61169/1990 and Japanese Utility Model Laid-Open No. 61170/1990.
In the case of an even-numbered laminate metal gasket such as a four-layered metal gasket, a structure wherein two bead plates having a projection form so that their bead portions are spaced apart progressively from each other are laminated and two of the resulting laminates are superposed one upon another can be employed.
In the case of a laminate gasket consisting of five metal plates, a flat metal plate not having a bead portion is sandwiched at the center of the four metal plates.
In the metal gaskets having the structure described above, the metal plate having the bead portion is shaped in such a manner that its bead is curved at and rises from the flat plate portion under a free state, then inclines slantwise, is bent in an opposite direction and extends once again in parallel with the flat plate portion.
The metal gasket of the type described above wherein a predetermined number of thin metal plates are laminated has high flexibility and high heat-resistance and exhibits suitable compressibility when a bead structure is provided thereto. When the metal gasket equipped with the bead plates having such bead portions is clamped and tightened between the joint surface of the cylinder head and the exhaust manifold by fastening bolts, or the like, the bead portions define a loop-like seal surface to the contact surfaces of the cylinder head and the exhaust manifold and prevents effectively an exhaust gas from leaking from such opposed surfaces.
Such a metal gasket is economical and is less likely to undergo degradation of its seal performance. When used as a gasket for an exhaust manifold which is exposed to high temperatures and whose coefficient of thermal expansion increases, the metal gasket is extremely advantageous from the aspect of seal performance.
Contrary to the advantage described above, however, the conventional metal gasket is not free from the following drawbacks. In other words, as higher and higher performance of engines such as compactness in size, reduction of weight and higher output has been required in recent years, a cylinder head is made of an aluminum alloy to reduce the size and weight and to reduce the thickness of an exhaust manifold and the temperature of an exhaust gas is raised to a higher temperature as one of the means for accomplishing higher engine performance. If the cylinder head is made of an aluminum alloy, thermal expansion or thermal shrinkage of the cylinder head increases due to the temperature change with an engine operation and if the thickness of the exhaust manifold is reduced, the quantity of thermal deformation of a gasket fitting surface or in other words, a flange portion, increases. If the exhaust gas temperature is raised to a higher temperature, thermal deformation of the exhaust manifold is further promoted by its temperature rise.
These conditions become severe for the metal gasket when a thermal stress and a mechanical stress are taken into consideration and the critical limit at which the metal gasket can follow the opposed surfaces of the cylinder head and the exhaust manifold where thermal deformation becomes great must be pursued.
With the background described above, severer requirements for a high temperature gasket have been imposed on the metal gasket for a manifold.
If a stainless steel sheet is used for the metal gasket, therefore, the material itself has high durability inclusive of heat-resistance but when exposed to thermal fluctuations having a large width of temperature change, it is not free from the following problems. Namely, the temperature difference between the cylinder head and the exhaust manifold changes remarkably in accordance with various operating conditions of the engine inclusive of the stop of the engine and the difference in thermal expansion between the cylinder head and the exhaust manifold, that is, the relative displacement distance, increases and invites permanent strain where the height of a residual bead at the bead portions of the metal gasket decreases or cracks of the bead portions resulting from the repetition of stress on the metal gasket. All of them result eventually in degradation of seal performance such as the leak of the exhaust gas.